1. Field of the Invention
The present invention relates to a polarity switching short circuiting arc welding method for ensuring efficient switching between electrode positive welding and electrode negative welding during a welding operation.
2. Description of the Related Art
In consumable electrode short circuiting arc welding, a consumable electrode provided by a welding wire is supplied at a constant rate and the welding proceeds in a repeated cycle of an arcing stage and a short circuiting stage between the welding wire and the base metal. The category of short circuiting arc welding includes CO2 welding, MAG welding, MIG welding, etc. which use a medium or low range electric current. In the short circuiting arc welding, normally, the welding is made in electrode positive polarity in which the wire serves as a positive electrode while the base metal serves as a negative electrode. However, when a work is thin and has a large gap, the welding may be made in electrode negative polarity in which the welding wire serves as a negative electrode while the base metal serves as a positive electrode. Electrode positive welding enables deep penetration and stable welding over the entire range of electric current. On the other hand, electrode negative welding enables to reduce heat input in the base metal as well as to increase the amount of molten wire. For this reason, it is possible to achieve a high level of quality in the welding of sheet plate works spaced at a large gap. For both polarities, a constant voltage power source is used.
Switching the output polarities of the welding power source not during welding but while welding is stopped can be made by providing a switching element or a mechanical switch in the welding power source output circuit. Since no welding is going on, the polarities may be switched at any timing. On the other hand, if the polarities are to be switched during welding, many arrangements must be made as will be described here below. Switching the polarities during welding enables to achieve higher welding qualities when work's joints and thickness change along the welding line. In such a case therefore, there will be several times of polarity change during a single cycle of welding. When switching polarities during welding, if the switching is made during an arcing stage, the switching will cause an arc extinction, which results in a defective welding. In order to prevent this arc extinction, a high voltage of about a few hundreds of volts must be applied at the time of the polarity switching. The circuit for this high voltage application has a problem that it is complex, and so expensive, as well as large. In order to solve this problem in short circuiting arc welding, there is a prior art disclosed in JP-A-S58-38664 to be described below. This prior art makes sure that polarity switching during welding is made in a short circuiting stage. Because the switching is made out of the arcing stage, there is no problem of arc extinction, and therefore there is no need for the arc re-striking circuit. Hereinafter, this conventional technique will be outlined with reference to a drawing.
FIG. 5 is a voltage-current waveform chart in conventional polarity switching short circuiting arc welding. Part (A) of the figure shows a polarity switching signal Sa, part (B) of the figure shows a polarity switching start signal Sb, part (C) of the figure shows a welding voltage v which is a voltage between the welding wire and the base metal, and part (D) of the figure shows a welding current i which passes through the arcing/short circuiting load, respectively in the form of time course changes. In this figure, a symbol EP indicates electrode positive polarity whereas a symbol EN indicates electrode negative polarity. Hereinafter, description will be made with reference to this figure.
During a short circuiting period Ts, as shown in part (C) of the figure, the welding voltage v takes a short circuiting voltage value of around several volts, and as shown in part (D) of the figure, the welding current i increases gradually. Following this, during an arcing period Ta, as shown in part (C) of the figure, the welding voltage v takes an arc voltage value of around a few tens of volts, and as shown in part (D) of the figure, the welding current i reduces gradually. The above-described state is generally the same whether in the electrode positive polarity EP or the electrode negative polarity EN.
At a time t1, as shown in part (A) of the figure, the polarity switching signal Sa changes. In response to this, as shown in part (B) of the figure, the polarity switching start signal Sb changes at a time t3 which is after the first short circuit occurred at a time t2 after the time t1. In response to this, the welding voltage v shown in part (C) of the figure and the welding current i shown in part (D) of the figure switch from electrode positive polarity EP to electrode negative polarity EN. In the prior art, polarity switching is made during the short circuiting period, but nothing is specified as to the timing of switching during the short circuiting period.
According to the prior art, polarity switching is made at the time t3 during the short circuiting period as shown in FIG. 5, and therefore, it is possible to prevent arc extinction without an arc re-striking circuit. However, as shown in part (D) of the figure, the current value ip at the polarity switching time t3 is as high as a few hundreds of amperes. Since this large current value ip is cut quickly by a switching element or the polarity switching, the switching element is subject to a very large surge voltage exceeding 500 V. In order to protect the switching element from this surge voltage, a special surge voltage protection circuit is necessary, and since the surge voltage protection circuit uses expensive parts, there is a problem that it increases the price of welding power source.
FIG. 6 is a voltage-current waveform chart showing a polarity switching which is made at a time point t2 when the first short circuiting occurs after the polarity switching signal Sa has changed. The figure corresponds to the above-described FIG. 5, differing only in the operation after the time t2. Hereinafter, description will be given for an operation after the time t2.
At the time t2, when polarity switching start signal Sb changes simultaneously with the occurrence of the short circuiting as shown in part (B) of the figure, a polarity switching operation starts. If the short circuit opens incidentally and an arc re-strikes at this moment, as shown in part (C) of the figure, the welding voltage v increases from the short circuiting voltage value to the arc voltage value. Since the polarity switching proceeds in this arcing stage, an arc extinction occurs, and as shown in part (C) of the figure, the welding voltage v becomes a no-load voltage. Thus, as shown in part (D) of the figure, the welding current i ceases. Right after the short circuiting, the current value at polarity switching moment ip is so small that the surge voltage is small. However, the short circuiting stage right after the short circuiting is not yet firmly established, and so the arc is likely to re-strike. There is no consistency as to the timing of arc re-striking during the short circuiting period, so arc can re-strike in the polarity switching in FIG. 5. If polarity switching is made when arc is re-striking, there is a high probability for arc extinction unless the system is provided with an arc re-striking circuit. As has been described, technical challenges in the prior art includes reduction of surge voltage incidence and arc extinction incidence accompanied by arc re-striking during polarity switching, without providing an arc re-striking circuit or a large capacity surge protection circuit.